Preventing Mold Growth in Below-Ambient Systems

Steve Fisher

July 1, 2014

Below-ambient systems create unique environments that have the potential for a multitude of issues, one of which is mold growth. Mold growth can occur either on or in
mechanical insulation on
pipes and tanks if the right conditions are present. When mold grows inside duct work, it can be a very significant issue, and cause for concern. The conditions in ducts
can be conducive for mold growth and the potential for health issues is high, particularly in schools, public buildings, and health-care facilities. Selecting the correct
insulation type and installing it
properly according to industry specifications and the manufacturer’s instructions can significantly decrease the probability of mold becoming an issue.

The Risks of Mold Growth

In the past 15 years, the concern over mold has increased in large part due to lawsuits. Many observers wondered if mold would become the next big industry issue and be
the subject of an
increasing number of lawsuits and other litigation. It is important to remain vigilant about the risks of mold growth. When mold problems arise in the field, the issues
relating to them generally
become the responsibility of the Design Engineer or the Installation Contractor. Correcting a mold problem can cost thousands, if not millions, of dollars. When mold is
found in the walls of a
multi-story condo complex because the insulation on the cold water lines did not perform properly, the cost to repair the damage can be particularly high. If mold is found
in the duct system of a
new elementary school, the cost of remediation can also be very high, but can reduce health risks to the occupants. These real world examples should make every Design
Engineer,
Mechanical Contractor, and Insulation Contractor take the issue of mold very seriously.

Environmental Elements Needed for Mold Growth

To eliminate mold, it is crucial to understand the key environmental elements required for it to form. For mold to form and thrive, 3 conditions must be present: an
environment with an
air temperature within a range typically found in building interiors, the presence of a food source, and the presence of either sustained high humidity or water. The best
way to
attack mold is to eliminate 1 or more of the 3 conditions necessary for its growth. Elimination of the air temperature conditions is usually not an option. Eliminating the
food source can
be difficult, but should be considered. The food source does not have to be the insulation itself, which is often blamed. Rather, it can be the dirt or dust attached to the
insulation, which
is almost always present. One option for reducing the propensity of the insulation to attract or capture dirt or dust is to use a jacket or select an insulation material
that resists dirt
accumulation. However, use of a jacket in a duct liner application is usually not a viable option, so it can be difficult to completely eliminate the dust or dirt that
serves as a food
source in those applications. Another option is the installation of UV lights inside ducts to eliminate mold—though this merely treats the mold rather than
eliminating the source.

Reducing Moisture to Prevent Mold Growth

The most common and tenable option for eliminating mold growth is to reduce moisture. Moisture can exist as either high humidity conditions or as water. Systems that
operate at below-ambient
temperatures have the potential for surface condensation, making them susceptible to mold growth. The use of biocides can be used as a secondary deterrent, but eliminating
the moisture
required for mold growth is the best option. There are several steps that can be taken to eliminate the presence of moisture, which can prevent mold from becoming an issue.
Three key steps are
1) specifying an adequate insulation thickness to prevent surface condensation; 2) selecting insulation materials that do not provide a food source for mold; and 3)
installing the
insulation correctly and maintaining the insulation system.

Step 1: Specifying Proper Insulation Thickness

In a below-ambient system, the first step in preventing mold is specifying a thickness of insulation that will prevent surface condensation most of the time. When
determining this, it is
important to remember that codes or other minimal specified levels of insulation—even for below-ambient lines—are generally developed for thermal efficiency,
not condensation control.
Preventing surface condensation sometimes requires greater thicknesses than those specified just for energy conservation (i.e., usually based on an economic thickness). To
determine design
conditions for unconditioned spaces (i.e., those spaces that are not continuously controlled for air temperature and percent relative humidity), it is recommended that the
designer follows the guidelines given in industry resources such as the ASHRAE 2013 Handbook of Fundamentals, Chapter 23, and the Mechanical Insulation Design Guide.

Step 2: Selecting Mold-Resistant Insulation

Unfortunately, surface condensation cannot be totally eliminated from ever occurring for most systems. Some condensation will probably occur in the rare situations when
the environmental
conditions exceed the design conditions for which the insulation thickness was specified, particularly in unconditioned spaces. Selecting a jacket for the insulation and
specifying an insulation
that inherently resists moisture penetration will reduce the probability of condensation. Using a jacket that incorporates a metal foil in its construction, does not have
exposed
paper on its outer surface, and is sealed tightly at the joints is a good first step.

Selecting an insulation that is resistant to mold growth is the second step in preventing mold and reducing the issues casual condensation creates. Insulations with a
closed-cell structure—such as elastomeric, polyethylene, polystyrene, or cellular glass—are resistant to moisture penetration and have a history of resisting
moisture without the need for an additional jacket. Other insulation materials work well with the proper design, vapor barriers, or jacketing. The best insulation to use in
a specific application (i.e. cold water, chilled
water, refrigeration, duct liner, etc.) depends on the temperature, configuration/shape, and specific requirements of the application. Some insulation materials have a
biocide incorporated in them for added mold resistance. The biocide should be approved by the U.S. Environmental Protection Agency (EPA) for use in ducts (air stream).

There are several test methods used to determine if an insulation or jacket material is “mold resistant.” In all cases, the test methods use clean, dry insulation or
jacket samples, which is
usually not comparable to real world conditions where dirt/dust is nearly always present as a food source. An insulation material’s testing and certification that it is
“mold resistant,” even when tested in accordance with a specific ASTM mold-resistant method, does not guarantee a mold-free installation if other factors are ignored. These
various tests and mold-resistant certifications may be useful, however, when comparing different insulation materials—while insulation cannot be guaranteed to be mold
free, some materials may fare better than others. When comparing
insulation materials, the same exact test method should be used for the basis of comparison. There are several third-party laboratories that certify products as being mold
resistant and are often
cited on insulation material product data sheets.

Step 3: Properly Installing and Maintaining the Insulation

Installing the insulation and establishing a proper maintenance plan is the third step in preventing mold in a below-ambient system. The insulation materials being
installed should be dry,
and should have never been exposed to water. The building should be enclosed (i.e., not exposed to the elements), which limits the potential for the insulation being
exposed to water once
it is installed. In addition, when installing insulation on an HVAC system, the system should not be running and the piping and ducts should be dry. While installing the
insulation, it is
mandatory for the installer to maintain the integrity of the insulation system for proper performance. Sealing all seams (butt, longitudinal, and termination points) is
mandatory for
below-ambient applications. Some insulation materials, such as flexible elastomeric and polyolefin, may use a contact adhesive rather than a tape to seal the seams. If the
insulation
becomes damaged (tears, holes, cuts, etc.), it must be repaired in a timely manner. Damaged insulation can lead to the degradation of the insulation on the whole system. It
is ideal, and less
costly, to both design the application properly and install it according to the manufacturer’s instructions than it is to make the repairs after it is installed.

Following the aforementioned steps and making everyone working on the project aware of mold as a potential problem can often prevent it from occurring. The key to
preventing mold
growth is simple: keep it clean and dry. Effective condensation control to eliminate the presence of water is not a matter of chance, compromise, or cost minimization. From
the beginning of the
project, it requires clear communication among all parties involved and clear instructions on what is expected of them. While some condensation may occur when environmental
conditions exceed the
design conditions for which the insulation is specified, it is possible—with proper planning, materials, and maintenance—to materially lessen the chance of
condensation and the
conditions that may lead to mold growth.